This invention relates generally to means for automatically determining the predominant frequency of a unique but unknown time varying signal over a large frequency spectrum containing the unknown predominant frequency. More particularly, the invention relates to apparatus and a method for searching a broad frequency spectrum to determine and verify the predominant unique unknown frequency.
In systems where the signal output of a variable frequency oscillator is intended to find, track and lock-on to both the phase and frequency of a predominant unknown second signal, it is a system requirement that an error control signal be developed that is proportional to the phase displacement between the two signals to enable adjustment of the oscillator output signal to effect the lock-on and tracking. If the predominant frequency of the second signal is basically unknown, complications arise because of the possible presence of ambiguities caused by the many seemingly stable nulls at harmonics and sub-harmonics of the oscillator output signal. Under such conditions, recognition of the correct phase and frequency of the second signal becomes very difficult to automate.
While the need for automating such a system is present in many applications, the need is readily apparent when one considers the need for pitch matching more or less exactly, the effective width of a scanning sensor to the pitch of plural stacked material to be counted by the article counting apparatus disclosed by WILLIAM L. MOHAN, et al in U.S. Pat. No. 4,373,135, issued Feb. 8, 1983 and by SAMUEL P. WILLITS, et al in U.S. Pat. No. RE27869 issued Jan. 1, 1974. The improvements of this invention are particularly useful when applied to means for effecting a substantially exact pitch match of effective sensor width to the pitch of a plurality of substantially identical objects stacked adjacent one another to achieve the equivalent of either a single-line pair or multiple-line pair sensor arrays effectively pitch matched.
In both of the aforementioned patents, the importance of maintaining a particular relationship between the sensor arrays effective width and stacked object thickness was explained and means were described for signal enhancement for conditions of low signal amplitude such as occur where low contrast gradients between adjacent stacked objects are encountered. While the apparatus of the MOHAN, et al patent, succeeded in overcoming many of the limitations of the earlier WILLITS, et al patent, the resulting system was still subject to counting errors when an operator incorrectly adjusted the system input parameter relating to object width by more than approximately fifty percent either over or under. For input errors that were less than such a 2:1 range, the compensation achieved by the pitch match and velocity correcting systems of the MOHAN, et al, invention provided adequate correction but, that invention has no systems to correct for input errors in excess of that approximate 2:1 limit on the range of input pitch data. Mismatch errors are frequently encountered and particularly where set-in values are left to human judgment rather than actual measurement, automatic correction of such errors is desirable and often necessary.